Blast induced neurotrauma (BINT) has been designated as the “signature injury” to warfighters in the recent military conflicts. In the past decade, conflicts in Iraq (operation Iraqi freedom) and Afghanistan (operation enduring freedom) as well as the increasing burden of the terrorism around the world resulted in an increased number of cases with blast Traumatic Brain Injury (bTBI).
Recently, a lot of research has been done to study the neurological and neurochemical degenerations resulting from BINT using animal models especially rat models. However, it is not clear how and whether the biological outcomes from animal models can be translated to humans; this work is aimed to address this issue.
In this dissertation, the criteria for achieving a standardized methodology to produce shock blast waves are identified.Firstly, shock tube adjustable parameters (SAPs) such as breech length, type of gas and membrane thickness were used for controlling and producing desired blast waves by manipulating shock wave parameters (SWPs). Secondly, using a surrogate head model, the data from the laboratory experiments were compared with experimental data obtained from the field explosions data to show the validity of the laboratory experiments. Finally, effect of test section location on the fidelity of the rat model in simulating field conditions was studied. Through these steps a standardized and accurate method of replicating the field blast was established.
Using the standardized methodology to model blast waves, the intracranial pressure for various incident pressures on the rat model was studied. Furthermore, to understand the mechanisms of loading and to study the influence of field variables, a finite element model of rat along with the simple ellipsoidal model was developed. With these models, the variables that influence the intracranial pressure such as skull thickness, skull modulus,and skull shape and skull cross section area were studied.
Finally, experimental data of intracranial pressure from rat and postmortem human specimen (PMHS) along with their corresponding numerical models were used to develop a model to predict the intracranial pressure. Finally, from this model it was predicted that for the same incident pressure human sustain a higher intracranial pressure than rats, which is contrary to the current scaling law developed to scale injury threshold across species, based on mass.
Blast Injury and Blast Induced Neurotrauma in Recent Wars:
The classification of blast injury, when an explosive of mass W goes off with a subject at a standoff distance r. The subject may suffer one or more of the following: (a) primary injury due to direct impingement of the shock blast wave, (b) secondary injury from interaction with shrapnel and bomb fragments, (c) tertiary due to impact with environmental structures or/and (d) quaternary due to inhalation of toxic gases and also all the other injuries that is not included in the first three.
Explosions and Shock-blast Waves:
An explosion is a process of rapid physical or chemical transformation of a system into mechanical work. The work accomplished during an explosion is due to the rapid expansion of the gases formed at the time of the explosion.
The most essential sign of explosion is the rapid jump in pressure (in few microseconds) in the medium surrounding the source of explosion, which is known as the shock blast waves or simply blast waves. Explosion can be due to chemical (plastic explosives, IEDs) or physical method (shock tube, explosion of boiler, and powerful discharges such as lighting).
Compressed Gas Driven Shock Tube and Their Use For Simulating Blast Wave:
In the last section, a review about basics of chemical explosion including different phases of explosion and basic requirement for generating a shock was discussed. One of the main objectives of this work is to establish a standardized method to replicate field relevant blast conditions (without other artifacts of the field explosion) to study primary blast condition. In this section, we will review groups that currently utilize gas driven shock tube (physical method, i.e., no chemicals involved) for generating blast wave.
Computational Animal Models in BINT:
Unlike conducting experiments on post-mortem human subjects or human volunteers, conducting experiment son rats to understand TBIis easier. Therefore, unlike human computational models, animal computational model is not often used in the study of BINT. As a result, only a few computational animal models have been reported in the literature.
LABORATORY MODELING OF SHOCK-BLAST WAVE TO STUDY TRAUMATIC BRAIN INJURY RELEVANT TO THEATER
Experiments were carried out in the shock tube designed by our group and tested at the University of Nebraska-Lincoln’s blast-wave generation facility. The four main components of any compressed-gas driven shock tube are driver, transition, driven sectionsand catch tank.
The driver section (breech) contains pressurized gas (e.g., Nitrogen, Helium) which is separated from the transition by several frangible Mylar® membranes, while the driven section (including the expansion section) contains air at atmospheric pressure and room temperature.
Burst pressure is the pressure in the driver section (breech) at the time of the membrane rupture. This highly compressed gas when allowed to expand rapidly compresses the atmospheric air in the transition and driven sections generating a shock front. Burst pressure for different membrane thicknesses and breech lengths From, it can be seen that the burst pressure increases with an increase in the membrane thickness.
VALIDATION OF SHOCK TUBE THROUGH FIELD TESTING
It shows the 28in. and 9 in. shock tube. When the breech is pressurized, membranes rupture expanding the high-pressure gas to expand into the expansion section driving a shock wave into the test section.
Free-field (FF) Experiments:
RED head was subjected to blasts from live-fire explosives. Dummy was placed at 110in.(2794 mm) from the cylindrical C4 charge and 96in. (2438 mm)from the ground. The location of the dummy with the location of the explosive charge and the pencil gauges to measure incident pressure. Dummy was oriented with its anterior part facing the epicenter of the blast.
EFFECT OF PLACEMENT LOCATION ON BIOMECHANICAL LOADING EXPERIENCED BY THE SUBJECT
Materials and Methods:
Experiments were carried out in the 229 mm x 229 mm (9in.x9in.), cross-section shock tube designed and tested at the University of Nebraska-Lincoln’s blast wave generation facility. The three main components of the shock tube are the driver, transition, and straight / extension sections (includes test section). The driver section is filled up with pressurized gas (e.g. Helium) separated from the transition by several Mylar membranes.
Role of the APL on Biomechanical Loading
It shows incident pressure, pressure in the brain and thoracic cavity corresponding to various locations along the length of the shock tube. At APLs (a) and (b) incident pressure profiles follow the Friedlander waveform (Fig. 2.4 (b)) fairly well. Pressure profiles in the brain and thoracic cavity also have similar profiles (the shape is almost identical) to that of the incident pressure profiles. At these locations, peak pressures recorded in the brain is higher than the incident peak pressure and the peak pressure recorded in the thoracic cavity is equivalent to the incident peak pressure.
DEVELOPMENT AND VALIDATION OF THREE DIMENSIONAL RAT HEAD MODELS
Development of Rat Head Model:
Finite Element (FE) discretization
A three-dimensional rat head model was generated from the combined use of high resolution MRI and CT datasets of a male Sprague Dawley rat. This technique has already been used to develop realistic human head model from a series of MRI/CT images, and to develop two-dimensional model of rat brain.
Two different T2-weighted MRI scans (one for the muscle skin and other for the brain), and one CT scan (for the skull and the bones) were used. These three different scans were necessary to achieve proper contrast and segmentation of various tissues (i.e., muscle, skin, brain, skull, and bones).
Loading, Interface and Boundary Conditions:
As described in the experimental work on rat in chapter 5, model was subjected to blast in the frontal direction. As described by Ganpule in his work,there are two possible techniques to impose the shock conditions: technique (a) Modeling of the entire shock tube, in which driver, transition and extension sections are included in the model so that events of burst, expansion and development of a planar of the blast wave are reproduced; technique (b) Partial model with experimentally measured (p-t) history is used as the pressure boundary condition, where the numerical model comprises the downstream flow field containing the test specimen.
Application of the Numerical Model:
The validated model was used to study the blast induced loading on the rat. In this study, effect of increasing incident pressure is analyzed through the studying intracranial pressure and skull strains for carious skull modulus. Finally, the loading pathways induced due to blast interaction with head and the subsequent wave propagation on the brain was studied.
DETERMINATION OF VARIABLES THAT INFLUENCES SCALING OF INTRACRANIAL PRESSURE ACROSS SPECIES USING AN EXPERIMENTAL AND THEORETICAL APPROACH
Numerical Model for Parametric Study
To determine the variables that influence the ICP a parametric analysis using a simple ellipsoidal model consisting of skull (with rat skull properties) and brain (rat brain properties) was made. The details of the numerical model setup including loading and boundary condition.
The base dimension of the ellipsoid was 10.5 mm minor axis and 20.5 mm major axis as brain and skull is 2 mm thick surrounding the brain. 10-noded quadratic tetrahedral element was used for meshing both skull and brain and tie contact is established between the inner surface of the skull and the outer surface of brain.
Due to the increasing acts of terror as well as the asymmetric warfare encountered in theater, blast-induced neurotrauma (BINT) has become more prevalent. Exhaustive research efforts have been initiated recently to encounter this problem. Although, some progress has been made, much more work has to be done to increase the understanding of BINT.
Currently, a lot of research is conducted to study the biological consequences of BINT using animal model (especially rat). However, due to the varying mechanical and biological variables across species as well as nonstandard methods used for replicating field blast, it is impossible to translate any biological outcomes from animal model experiment to humans or to use the results in designing effective mitigation strategies. In this work, a standardized method for producing blast wave conditions(related to field conditions) pertaining to primary blast injury is proposed.
Furthermore, this knowledge along with numerical methods is used to study blast wave head interactions on rat model. Finally, a nonlinear regression model was developed to translate the incident pressure corresponding to injury from animal model to humans. Some of the contributions of this work are:
- It was shown that shock tube could be effectively controlled to produce blast wave profiles that are comparable to field explosion testing.
- The results from the field experiments and shock tube were compared to show that blast wave produced in the shock tube interacts with a surrogate in a similar manner seen in the field experiments.
- Experimental and numerical rat models were developed to study the blast wave interaction,intracranial pressureand skull flexure.
- A nonlinear regression model to translate the incident pressure between rat and humans was developed. With this model, the critical thresholds developed in rat model can be translated to humans. Furthermore, the thresholds deduced from the model can be used in developing personal protective equipments.
Recommendations are given based on the chapters. Some of the recommendations for the future work are:
- In the third chapter, only the positive phase of the blast wave was modeled in shock
tube and all the analysis to control and manipulate shock tube were done only on the
positive phase of the shock tube; however, the negative phase is also important and needs to be studied. It is hypothesized that negative phase or negative over pressure may be responsible for cavitation in the brain, which is an important injury mechanism that has to be studied.
- In the fourth chapter, where a comparison between field and shock tube experiments
were made, a lot more field experiments have to be done to reduce the type II error in the peak comparison of acceleration and over pressures. Furthermore, analysis to determine the high acceleration in shock tube compared to field experiments has to be finally, it should be determined whether these acceleration values affect the overall experimentation.
- In the fifth and sixth chapter, only intracranial pressure and direct transmission were
studied; however, other mechanisms such as cavitation, thoracic surge and injuries due to acceleration and deceleration has to be studied to have comprehensive understanding of the blast.
- In the final chapter, the number of data points used in the model should be increased. Furthermore, other species such as pig and mice should also be included to obtain a comprehensive model. Finally, limitations mentioned at the end of the chapter should be addressed.
Source: University of Nebraska-Lincoln
Author: Aravind Sundaramurthy